1. Field of the Invention
The present invention relates generally to mobile communications, and more particularly, to a method and an apparatus for transmitting a Hybrid Automatic Repeat reQuest ACKnowledgement (HARQ-ACK) in a Time Division Duplexing (TDD) Carrier Aggregation (CA) system, where distribution of a sub frame changes flexibly.
2. Description of the Related Art
A Long Term Evolution (LTE) system supports a working mode of TDD.
FIG. 1 illustrates a frame structure in a conventional LTE TDD system.
Referring to FIG. 1, each radio frame has a length of 10 ms and is equally divided into two half frames of 5 ms. Each half-frame includes 8 slots, each of which has a length of 0.5 ms, and 3 special fields, i.e., a Downlink Pilot Time Slot (DwPTS), a Guarding Period (GP), and an Uplink Pilot Time Slot (UpPTS). The 3 special fields have a total length of 1 ms.
Each sub frame (sub frame 0 to sub frame 9) includes two consecutive time slots, i.e., a k'th sub frame includes a time slot 2k and a time slot 2k+1.
A TDD system supports 7 types of UpLink/DownLink (UL/DL) configurations, as shown in Table 1 below.
In Table 1, D represents a downlink sub frame, U represents an uplink sub frame, and S represents a special sub frame including the 3 special fields, i.e., a DwPTS, a GP, and a UpPTS.
TABLE 1LTE TDD UL/DL configurationConfigura-tionSwitch-serialpointSub-frame IDnumberperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
In an LTE TDD system, resource allocation is performed in units of Physical Resource Blocks (PRBs). Each PRB includes 12 consecutive sub carriers in the frequency domain and occupies one time slot in the time domain.
In a downlink sub frame, the first n OFDM symbols are used for transmitting downlink control information, which includes a Physical Downlink Control CHannel (PDCCH) and other control information, and the other symbols, i.e., the symbols following the first n OFDM symbols, are used for transmitting Physical Downlink Shared CHannel (PDSCH).
In subsequent versions of LTE, the concept of an enhanced PDCCH (ePDCCH) has been proposed. An ePDCCH is mapped to a data region in a sub frame to be transmitted. Accordingly, the ePDCCH and the PDSCH are implemented by Frequency Division Multiplexing (FDM). Herein, PDCCH and ePDCCH are not differentiated from each other, unless otherwise specified, and are commonly referred to as PDCCH.
A base station allocates PDSCH resources in one downlink sub frame through a PDCCH, which includes downlink grant signaling. A User Equipment (UE) receives and decodes the PDSCH after receiving the downlink grant signaling, and transmits a HARQ-ACK for the PDSCH in a subsequent uplink sub frame according to a certain timing scheme. Because TDD supports multiple types of UL/DL configurations, a HARQ-ACK for a PDCCH indicating downlink a Semi Permanent Scheduling (SPS) release or for PDSCH in 0 or 1 or multiple downlink sub frames may be fed back within an uplink sub frame n. The index of the downlink sub frames are n-k, where k belongs to a collection K, which is decided by the UL/DL configuration and the uplink sub frame n, as shown in Table 2 below.
TABLE 2index collection KUL/DLSub frame index nConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7,————8, 7,——4, 64, 63——7, 6,6, 55, 4—————114——12, 8,6, 5,——————7, 114, 75——13,———————12, 9,8, 7,5, 4,11, 66——775——77—
In an LTE-Advanced (LTE-A) system, multiple Component Carriers (CC) are aggregated to obtain larger working bandwidth, i.e., Carrier Aggregation (CA). The aggregated carriers constitute downlink and uplink links in the communication system, and therefore, larger transmission rates can be achieved.
A base station may configure a UE to work in multiple cells, which include one Primary cell (Pcell) and multiple Secondary cells (Scells).
When multiple aggregated cells use the same UL/DL configuration, HARQ-ACKs for all of the cells are fed back in an uplink sub frame of the Pcell, and the HARQ-ACK timing scheme defined in LTE Release 8 for a cell may be used, as shown in Table 2. However, when the aggregated multiple cells use different UL/DL configurations, HARQ-ACKs for all of the cells are fed back in an uplink sub frame in the Pcell, and the timing scheme according to one of the 7 conventional UL/DL configurations, as shown in Table 1, are used as the timing scheme for PDSCH in the Scells.
Table 3 may be used to determine the TDD UL/DL configuration whose HARQ-ACK timing scheme is to be used for downlink transmission of an Scell for each combination of a TDD UL/DL configuration of the Pcell and a TDD UL/DL configuration of the Scell.
TABLE 3determining TDD UL/DL configuration for HARQ-ACK timingReference UL/DLconfiguration for timingUL/DL configuration of Pcellof HARQ-ACK in Scell0123456UL/DL configuration of Scell00123456111244512222555233453453444544545555555566123456
In conventional LTE TDD standards, the UL/DL configuration used by a cell is configured via broadcast signaling, i.e., is included in a System Information Block 1 (SIB1). As such, the LTE system supports at least 640 ms between changes of UL/DL configurations and at most 32 changes of system information in every 3 hours according 3rd to conventional standards. To improve for fast changes in service characteristics, Generation Partnership Project (3GPP) is currently working on methods that support faster changes in distribution of UL/DL sub frames in the system. For example, a new method may change UL/DL configuration at a faster pace, e.g., every 200 ms; or change UL/DL configuration at intervals of 10 ms, which equals the length of a radio frame.
In practice, a base station scheduler may change the distribution of UL/DL sub frames according to service demands, and keep the system running properly by adopting certain scheduling restrictions. A UE does not have to know which of the 7 UL/DL configurations it is running under. Further, the distribution of working UL/DL sub frames is not necessarily limited to the 7 UL/DL configurations shown in Table 1, i.e., the actual distribution of UL/DL sub frames may be transparent to UEs.
In a CA system, when the distribution of UL/DL sub frames in some or all of the cells can change flexibly, both a base station and a UE need to correctly determine the timing of HARQ-ACK for PDSCH in each cell to allow the UE to correctly send a HARQ-ACK in an uplink direction and allow the base station to correctly receive the HARQ-ACK sent by the UE, such that downlink HARQ transmission can be implemented properly.